This application is a continuation-in-part under 35 U.S.C. .sctn.120 of prior copending application No. PCT/DE91/00642,which was filed on Aug. 10, 1991, and designates the United States.
The invention relates to a centering device for the feeler pin in a mechanical probe, comprising a driver, connected to the feeler pin, and two restoring means acting thereon in opposite directions.
A centering device of this kind is known from German Patent Specification 23 56 030, particularly claim 2 and FIG. 1.
In conjunction with the design of the driver as a flexural bar, it there serves primarily for the variation of the spring constant in dependence on travel.
At its end the driver carries a ball. Springs with the same spring constant act on that ball on both sides through plane pressure members. An arrangement of three balls acts on both pressure members independently of one another as first and second stops.
In order to ensure faultless centering, free from hysteresis, in the normal position, all contact points provided between the driver ball, the balls of the stops, and the pressure members must be in accurate contact at the same time.
This is possible only with heavy expense for manufacture of precision balls and optical quality planeness of the pressure members.
During use this quality is necessarily impaired by wear, by the action of impacts and friction between the driver ball and the pressure members.
The plane pressure members of the restoring means always lie in frictional contact with the driver ball head. This frictional contact gives rise to frictional forces between the surface of the ball and the plane pressure surface when the driver deflects and when it is returned to its normal position.
The driver is deflected through the action of external forces which, because of the lever arm of the feeler connected to the driver, are considerably greater than said frictional forces. The forces for the return to the normal position must however be applied by spring forces within the probe, which are composed of the restoring forces of the centering device and the spring constant in the case of a measuring feeler. These forces cannot be increased at will, since they have a direct effect on the pressure with which the feeler is pressed against a workpiece, in respect of which limits must be set in order to avoid damage.
The frictional forces must therefore be taken into account in the design, and particular attention must be paid to the transition from sliding friction during the return movement to the stationary friction in the normal position. In a concrete example of construction of a measuring feeler repeated measurements have shown that the reproducibility of the normal position of the driver, and therefore the zero position of the feeler, is burdened with an uncertainty of about .+-.0.4 .mu.m.
The problem of zero position uncertainty in probes of the switching type is known per se from DE 38 24 548 A1. As a solution it is proposed to cause the feeler to vibrate during a predetermined period of time after each probing operation. The aim is thus to "shake" the feeler into its bearings, particular mention being made of the excitation of a piezoelectric oscillator as the source of the shaking action. The oscillation imparted to the feeler for a short time after it has dropped back into its bearings converts the stationary friction into motional friction, which is considerably lower, so that the restoring forces of the feeler system are sufficient for further centering in the bearing. These interrelationships are general knowledge in precision mechanics, particularly for lens centering by the bell clamp method (DE 1 004 516 B1).
The problem underlying the invention is therefore that of providing a centering device of the type defined, which, while having the same precision, can be produced at markedly lower cost. Increased hysteresis due to wear during protracted use should not occur. In addition, zero position accuracy should be increased, particularly in the case of use in measuring feelers.